Typical petroleum drilling operations employ a number of techniques to gather information about the borehole and the formation through which it is drilled. Such techniques are commonly referred to in the art as measurement while drilling (MWD) and logging while drilling (LWD). As used in the art, there is not always a clear distinction between the terms LWD and MWD. Generally speaking MWD typically refers to measurements taken for the purpose of drilling the well (e.g., navigation) and often includes information about the size, shape, and direction of the borehole. LWD typically refers to measurement taken for the purpose of analysis of the formation and surrounding borehole conditions and often includes various formation properties, such as acoustic velocity, density, and resistivity. It will be understood that the present invention is relevant to both MWD and LWD operations. As such they will be referred to commonly herein as “MWD/LWD.”
Transmission of data from a downhole tool to the surface is a difficulty common to MWD/LWD operations. Mud pulse telemetry is one technique that is commonly utilized for such data transmissions. During a typical drilling operation, drilling fluid (commonly referred to as “mud” in the art) is pumped downward through the drill pipe, MWD/LWD tools, and the bottom hole assembly (BHA) where it emerges at or near the drill bit at the bottom of the borehole. The mud serves several purposes, including cooling and lubricating the drill bit, clearing cuttings away from the drill bit and transporting them to the surface, and stabilizing and sealing the formation(s) through which the borehole traverses. In a typical mud pulse telemetry operation, a transmission device, such as an electromechanical pulser or a mud siren located near the drill bit generates a series of pressure pulses (in which the data is encoded) that is transmitted through the mud column to the surface. At the surface, one or more transducers convert the pressure pulses to electrical signals, which are then transmitted to a signal processor. The signal processor then decodes the signals to provide the transmitted data to the drilling operator.
One significant difficulty with decoding a mud pulse signal is the poor signal to noise ratio that results from both low signal amplitude and high noise content. Due in part to the poor signal to noise ratio, data transmission rates are slow (e.g., on the order of about 1 to 10 bits per second). Increasing the transmission rate tends to decrease the signal to noise ratio due to decreased signal amplitude. The low signal to noise ratio also tends to increase the frequency of transmission errors which can erode the reliability of the communication channel and disrupt the synchronization between the downhole encoder and the surface decoder.
The amplitude of a transmitted pressure pulse tends to attenuate as it travels up the drill pipe. Such attenuation typically depends on many factors including the depth of the borehole, the type of drilling mud, the number of joints in the drill string, the inner diameter of the drill string, and the frequency of the signal. Moreover, there are a number of potential sources of noise generated during drilling operations including turning of the drill bit and/or drill pipe in the borehole, sliding and/or impact of the drill pipe against the borehole wall, and the mud pump that is used to pump the mud downhole. Mud pump noise tends to be particularly troublesome since the detectors are located at the surface close to the pumps and since the pump noise is typically strong in the frequency range commonly used for data telemetry (e.g., between 1 and 20 Hz).
Distortion of the signal may also be introduced by reflections from the ends of the mud channel, from tool joints or other diameter changes in the drill string, and from dispersion or filtering of certain frequencies within the mud channel. A strong reflection is often observed at the detectors from the uphole end of the mud channel (e.g., from the mud pumps and the pulsation dampeners).
Given the difficulty inherent in mud pulse telemetry operations, there have been numerous efforts to improve the communication channel. For example, Umeda (U.S. Pat. No. 4,642,800) discloses a method in which the measured signals are averaged to produce an average signature signal. The average signature signal may then be subtracted from a current signal so as to remove the noise component and produce a residual signal which contains the data component. One byproduct of this technique is that the pump signature may be estimated.
Various attempts have also been made to remove reflections from the measured signal via the use of first and second spaced transducers. In these attempts, the signal measured at one receiver is generally delayed by the approximate time it takes the pressure wave to travel between the transducers and then subtracted from the signal measured at the other transducer. Such methods have been disclosed by Garcia (U.S. Pat. No. 3,742,443); Rodney (U.S. Pat. No. 4,590,593); and Scherbatskoy (U.S. Pat. No. 4,692,911). Difficulties associated with the removal of reflections include a precise determination of the appropriate time delay, attenuation and/or distortion of the signal between the two transducer locations, and deconvolution of the resulting difference signal to recover the original signal.
Attempts to solve these problems have generally used the received signals to determine a transfer function of the communication channel between the two transducers. Chin (U.S. Pat. No. 5,969,638); Abdallah (U.S. Pat. No. 6,308,562); Fincher (U.S. Pat. No. 7,313,052); Reckmann (U.S. Pat. No. 7,423,550); and Wasserman (U.S. Patent Publications 2008/0002524 and 2009/0016160) disclose such methods. While these approaches enable a transfer function to be estimated they are not without certain drawbacks. For example, a transfer function computed using received signals can be unreliable (and inaccurate) since the received signals include components traveling in both the uphole and downhole directions (e.g., the telemetry signal travels in the uphole direction, while pump noise travels in the downhole direction). An unreliable transfer function results in a misrepresentation of the signal attenuation and/or distortion and therefore can in turn result in telemetry errors.
Therefore, there exists a need for an improved method for reducing noise and other unwanted signals in mud pulse telemetry operations.